|
Basic
Theory
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Widely
popularized by Acoustic Research back in the 50’s and continues to
make its way into several popular commercial and DIY loudspeaker
designs today.
The driver is mounted in a sealed, airtight
enclosure, generally with the front of the driver facing outward
but is not restricted to this method only. The
volume of the enclosure is chosen to achieve a
specific desired system Q which defines the response characteristics
of the driver and enclosure. Q values range between the
0.5 and 1.5 - with 0.5 being overdamped, 1.5 being
underdamped, and 0.7 being critically flat. The total system
Q (also known as Qtc)
is dependent
on 3 things: the volume of enclosure, the T/S parameters of driver and
internal
treatment compounds. A sealed enclosure is best suited for drivers with an EBP
(Efficiency Bandwidth Product) of 50.0 or lower
and drivers with Qts values above 0.40 but is not restricted to
these exact alignments. EBP is calculated by taking the the
fs of the driver and dividing it by the Qes - therefore EBP = fs/Qes.
The cutoff rate is typically 12
dB/octave below f3, however higher system Q's result is a
somewhat sharper roll-off (~14 dB/octave) while lower system Q's result in a
slightly more shallow roll-off (~10 dB/octave).
Better damping and better transients is achieved by shooting for a lower system Q which
can be accomplished by either making the enclosure larger or by
adding stuffing/damping material. Suitable damping materials
include polyfill, Dacron, fiberglass, and acoustic foam. Box
stuffing will also affect f3 by either raising it or lowering it
depending upon the type and amount of stuffing used.
Stuffing makes the box "appear" to be acoustically larger than it
really is. |
|
Advantages |
2nd order sealed
enclosures are the
simplest to design yet still offer outstanding performance and can
therefore be the most rewarding.
They are easy to model with software and it is easy to achieve
predicted results. Box size and shape are generally the least complex. Great for both beginning and advanced DIY’ers.
The
exact desired response characteristics can be achieved
by simply designing for a particular Qtc (or system Q).
Modeled performance is easily altered by varying the size of the
enclosure and the amount of stuffing material used.
They exhibit a very shallow cutoff rate of
12 db/octave below fB.
This results in much better group delay response that may
range from 1ms – 10
ms. Fast,
quick, natural, smooth, tight, accurate, controlled and warm are
some common subjective terms one might use to describe sealed
enclosures. Transient response is the best of all enclosure types.
The excursion of the driver increases as the
frequency applied decreases until fB is reached after which the
driver excursion begins to decrease.
There is no
need for subsonic filtering due to the enclosure’s natural
tendency to inhibit extremely low frequencies. This results in less bottoming out of
drivers at subsonic frequencies.
However, this only applies for smaller enclosures. As the enclosure size gets larger, more Xmax
is required in order to prevent overexertion for the same amount of input power.
Sealed enclosures have more extended low frequency response than vented enclosures given
the same f3 for both due to the shallower rate of roll-off. Phase shift is minimal within its normal operating
frequency range.
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|
Disadvantages |
Very low frequency output is
difficult to achieve without active filtering.
The f3 (also know as 3dB down point) is usually fairly
high, above 30 Hz in most applications and by simply increasing Vb,
one cannot
lower f3 for any given driver. Low f3's in a sealed enclosure
can be achieved by using drivers with a very low free air resonance
or Fs.
Less power efficiency by about -3 dB as compared with
vented enclosures. Lower
over SPL capabilities.
There's a strong
need for drivers with a very large Xmax in order to ensure safe
operation at least down to fB, especially if the box is
designed for Qtc values < 0.7 Any enclosure volume
that is modeled with the system Q larger than 0.707 results in higher
f3. Lowest f3 is achievable only under an ideal Q = .707
alignment which may require unusually large and sometimes
unacceptable enclosure volumes.
|
|
Best
Applications |
Best suited where
a completely uncompromised sound quality is desired. Best for classical music and most rock and pop type music.
Most widely used in car stereo systems where cabin gain can
make up for its lack of low end <30Hz bass. Where size is an issue.
Sealed boxes can be half the size of vented boxes yet
can be made even smaller if a higher Q is allowed. May also be use for small to moderately sized Home Theaters.
Usually is the easiest box to pass SAF (spouse acceptance
factor). You should also go with sealed when the driver's
T/S parameters dictate that the driver should be housed in a
sealed enclosure due to a high Qts (above 0.4)
or an EPB of 50 or lower - though this just a guideline and
not a rule. |
Ported - 4th
Order Vented
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Basic
Theory
|
Also
known as bass-reflex, ported or vented. The
driver is mounted into an enclosure which houses a large
opening, port, vent or slot that extends into the cabinet a
specified length. The
length and area
of this vent are extremely critical to the proper function of a
4th order enclosure. The port and
driver contribute together to provide the desired response
characteristic. The driver
is generally mounted with the front facing outwards, but is not
restricted to this method only. The
vent which extends into the cabinet tunes the enclosure to a
specific frequency (known as fB) thereby acting as a high pass
filter on the driver. Driver
excursion is at its minimum at fB where the vent then takes over
and provides most of the output.
Cut-off rate below fB is 24 dB/octave but can be varied up
or down 5-6 dB depending upon the exact tuning frequency and
volume of the enclosure. There
are various types of alignments that all fit into the ported 4th
order category. Some
common types are QB3, EBS, SBB4 and SC4.
By varying the enclosure size and the tuning frequency, it
is possible to achieve a variety of distinct low- frequency
performances from a single driver.
The vent acts by damping the load produced by the driver
above fB causing it to behave somewhat as if it were in a sealed enclosure.
Best suited for drivers with an EBP near 100.0 or higher and Qts <
4.0 but is not restricted to these numbers only. |
|
Advantages |
Extended
low frequency response. 3
dB down points (f3) are capable of being near or even below 20 Hz.
Increased power handling above fB due to reduced driver excursion at and while nearing
fB. More
efficient system. Generally
3dB increased output over sealed enclosures due to the combined
output of driver and port.
More overall SPL capabilities.
Deep, powerful, full, loud, inspiring, incredible, and earth
shattering are common subjective terms associated with
vented enclosures.
|
|
Disadvantages |
Larger
enclosure size. More
difficult to accurately achieve predicted results.
Misaligned enclosures can result in very poor bass quality.
Very
accurate T/S parameters of actual driver is required. Although sometimes you can get away with using
manufacture’s specifications.
Driver unloading or bottoming out below fB is very common.
Xmax is reached easily below fB and may cause sever damage the the
driver's suspension, voice coil or cone. This usually requires the need to install additional high
pass filtering below fB. But is not a always a necessity as long as power levels and
frequency content are kept within reason.
Transient response is degraded, yielding typical group
delay curves as high as 50 ms.
Muggy, boomy, sluggish, one-note, slow, and inaccurate are common
subjective terms
associated with vented enclosures. Port diameter must be
large to avoid unwanted port noise, which in turn requires the
port to be long for any given Fb, which then drives up the volume
of the enclosure, sometimes to undesirably large proportions.
Port chuffing if port area is not kept in check. |
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Best
Applications |
Where
the deepest and loudest bass is necessary.
Where size is not a huge issue but may still be a definite
factor.
For Home Theater and music. May be best suited for sound
reinforcement, theater, live performances, DJ and other situations
where lots of loud deep bass is needed and transient response is
less critical.
|
Bandpass -
Dual Chamber Vented/Sealed 4th Order
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Basic
Theory
|
The front and the rear
of the driver are housed in their own separate and distinct
enclosures. The front of the driver is in a ported enclosure
while the rear of the driver is in a sealed enclosure. The
driver may be mounted the other way around however as long as one
chamber is sealed while the other is vented. The
enclosure is designed as a sealed enclosure but with the addition
of an acoustic filter (the port) in series with the front of the
driver that acts to limit the driver's bandwidth and therefore
increase its SPL capabilities within its bandwidth. |
|
Advantages |
Very low f3 is possible at
the expense of lower efficiency and increased ripple.
Extremely high SPL is also possible at the expense of a higher f3
and narrower bandwidth. Less overall driver excursion. More control over cone movement.
Bandwidth and efficiency are inversely proportional. |
|
Disadvantages |
Combined volume of both
chambers results in large overall enclosures. Difficult to design
properly. Results may vary substantially due to misalignment
of both front and rear chambers as well as tuning frequency.
Tend to have "one-note" bass, especially if designed or
built poorly. In order to achieve a wide useable
bandwidth, there must be some amount of mid-band ripple as well as
decreased efficiency.
Drivers can be easily blown due to high compression factors
because of lowered cone motion and thereby exceeding the thermal
limits of the driver before exceeding its mechanical limits.
Bandwidth and efficiency are inversely proportional. |
|
Best
Applications |
Where the large size of
enclosure is of little concern. In cars where the design
calls for high SPL where the limited bandwidth which results can
be increased due to cabin gain. The cabin gain will help achieve a flatter
and wider bandwidth across the desired range while maintaining the
increased SPL of the enclosure. Very popular in car
applications for this reason. |
Bandpass - Dual
Chamber Vented 6th Order
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Basic
Theory
|
The front and rear of the driver are
mounted in separate enclosures and tuned to specific calculated
values. Resultant output is suppose to be better than any
of the other designs mentioned previously. Bose owns
the rights to the exact details behind this design. They
explain the theory like this, "The low-frequency speaker
drivers are located between separate acoustic compression chambers
inside a patented Bose Acoustimass module. As each speaker cone
moves, it excites air in the chambers. Trapped in the chamber,
this air acts as an acoustic spring, which interacts with the air
in the port to produce more low-frequency sound with less power.
The system is more efficient and requires less cone motion, which
in turn produces less distortion. In the event that any otherwise
audible distortion is produced, the patented design traps it
inside the acoustic chambers -- so it never enters the room. The
result is an Acoustimass module with no audible distortion that
can be located anywhere in the listening area." www.bose.com |
|
Advantages |
More efficient system within its
bandpass. More control over cone movement.
Less audible distortion. This doesn't necessarily mean that there is a
true reduction in distortion from the driver, but that any distortion that is
present form the driver can't be heard as well due to the chambers
acting as filters on any unwanted noise. My opinion only. |
|
Disadvantages |
Combined volume of both
chambers may result in large overall enclosures. Very
difficult to design
properly. You may have to experiment for months before
getting this design to sound acceptable. Results may vary substantially due to misalignment
of both front and rear chambers as well as tuning frequency of
each chamber. Drivers can be easily blown due to high compression factors
because of lowered cone motion and thereby exceeding the thermal
limits of the driver before exceeding its mechanical limits.
The driver may in fact tear itself to pieces. There
are no exact parameters or calculations for designing 6th order
bandpass enclosures due to the patent owned by Bose. So if
you build one, you're basically on your own. |
|
Best
Applications |
For more information on 6th order
bandpass enclosures please visit www.decware.com |
EBS
- 4th Order Large Vented Enclosure with Low Tuning
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Basic
Theory
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EBS-
Extended Bass Shelf. This is only one of the various
different types of vented alignments which are possible and
follows many of the same characteristics of vented
enclosures. The idea is to intentionally design the
enclosure to be 125-175% larger than the optimal calculated volume
and then tune the enclosure much lower than optimal as well.
The result is a significant amount of extended low frequency
response. When the response curve is simulated, a visible
"shelf" can be seen in the curve just above the tuning
frequency before it sharply rolls off. The LEAP manual
explains EBS theory like this: "The name [EBS] was derived
simply from the visible appearance of the response curve. The bass
response is extended to a lower frequency than would be possible
from the QB3 alignment, but at a lower level or shelf relative to
the mid band level. Although the EBS alignment is not a nice neat
flat alignment such as the QB3, it is very often a much better
choice than the QB3. The EBS alignment has some interesting
features. Consider a loudspeaker with a Qts of 0.30, the QB3
alignment would have about 2dB more output at a frequency of twice
the Fs, while the EBS alignment would have over 2db more output at
Fs. In most cases the EBS alignments will have far more
subjective [low] bass than the QB3 alignments. Also, if you were
to equalize the responses flat to Fs, 10db more boost would be
required for the QB3 versus the EBS. This can dramatically consume
large amounts of headroom in the power amplifier, and may also far
exceed the linear excursion limits of the speaker. The EBS
alignment will maintain much lower cone excursion at frequencies
near Fb than is possible with the QB3 alignment. This can be very
important for high power systems." |
|
Advantages |
Extended
low frequency response down into the teens. Subsonic, earth-shattering bass response. Increased efficiency at the lower
frequencies (below ~25 Hz) but decreased efficiency at higher
frequencies (above ~30 Hz) depending on tuning and box volume. This is a rough figure since many
different combinations can be designed to yield specific
results. In general, low frequency is extended and
efficiency increased at the expense of reduced efficiency at
higher frequencies - all within the generally accepted range known
as bass. |
|
Disadvantages |
Cut-off
rate can be as high 36 dB/octave below fB. Transient
response is degraded as a result of this. However it may be
argued that because the tuning frequency is so low, that is it far
enough out of normal operating range that it may be considered a negligible
downside. The enclosure size is huge. Anywhere from
5-15 cubic feet depending on the size of the driver being used.
Power handling capability of driver is reduced anywhere from
25-50%. Driver may reach Xmax sooner above fB even if it
never reaches Xmax right at or below fB. Lack of real
presence, lack of kick or punch, may be subjective terms uses to
describe EBS alignments. The overall impact of
the bass is much softer. Signals between 40 and 60 Hz are
significantly reduced. Harder to "sell" because
most people are more receiving to a pronounced upper bass response
rather than an incredibly low and deep bass response . It
takes 8 times as much power (as well as moving air) to make 20 Hz
sound as loud as 40 Hz. |
|
Best
Applications |
Where the
truly deepest of all heavenly deep bass is desired. For
drivers with a large Xmax and the ability to consume large amounts
of power. For drivers whose T/S parameters dictate an
optimal enclosure size that's smaller than what the designer wants
to build. Large Home Theaters and varying kinds of music
with heavy bass tracks would take the best advantage of this
enclosure alignment. Where size is no concern and sub-20 Hz
bass is the main goal. Bragging rights. |
PR - Passive
Radiator
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Basic
Theory
|
A passive
radiator is used in conjunction with an active driver and its
purpose is to replace the port or vent of a typical 4th order
enclosure. A passive radiator is sometimes referred to as a
drone cone. A passive radiator (PR) enclosure is most
similar to a vented enclosure in that they acoustically behave very much the
same. Response characteristics include: there is a notch at
the Fp of the passive radiator (resonant frequency of PR) and the
typical cut-off rate below fB is 36 dB/octave. The resonant
frequency of the PR is intentionally altered by the designer in
order to achieve the proper fB of the enclosure. In other
words, it is used to tune the box to the desired frequency and for
optimum performance. This is done my adding or removing
calculated amounts of mass from the cone of the PR. More
mass = lower tuning. Less mass = higher tuning. The
increases mass also helps to lower the Fp of the PR which in turn
moves the undesired notch out of the passband resulting in
improved transient response. In theory, the Vd (volume
displaced by PR) should be at least 2 times the Vd of the driver
it is being used with. Yet in practice, a good rule of thumb
to go by is to have anywhere from 3-4 times as much displacement
in the PR. This is to ensure the longevity of the PR's by
preventing excessive continual over-excursion. |
|
Advantages
|
Simplicity
of tuning. By merely adding and removing small amounts of
mass, the tuning frequency of the
enclosure may be changed up or down by as much as 15 Hz or as
little as 0.1 Hz. Precision tuning is very possible.
Ability to tune small enclosures to very low frequencies without
the loss of volume due to internal ports taking up enormous
amounts of precious space. No port noise or any kind
of air-turbulence of air speed levels to worry about. Pipe
resonances and port standing waves are non-existent because there
are no ports or vents in this system. Better driver
stability below fB due to increased damping on the driver below fB.
This is because of specific compliance characteristics of the PR
which help to keep the driver under better control at subsonic
frequencies. |
|
Disadvantages |
Steepest
of all cut-off rates with a roll-off 36 dB/octave below fB.
Given the same size volume and tuning frequency of its vented
counterpart, the PR alignment would result in a slightly higher
f3. The expense of of PR's can be quite high when compared
to a simple piece of PVC pipe. Especially when 2 or more
PR's is needed, which is usually the case when using high-power,
high-excursion drivers. |
|
Best
Applications |
Best
suited just about anywhere a regular vented enclosure would be
used. May be used in applications where a smaller box is
desired while wanting to maintain a 4th order alignment. It
was once believed that the PR alignment can be made half the volume
of the same vented alignment and still be designed to have the same
tuning frequency and achieve the same frequency response.
Though this theory has been argued. Passive Radiator
alignments in most cases will require a comparable box volume and
tuning as 4th order vented alignment in order to achieve similar
results. |
TL - Transmission
Line
|
Basic
Theory
|
The driver
is mounted in a type of acoustical labyrinth or long pipe commonly
referred to as a transmission line. The length of this
transmission line is usually somewhere between 4-8 feet and is
dependent upon the Fs of the driver and the fill material used in
the labyrinth. This t-line may have a tapered effect
or maintain the same cross-sectional area throughout its length
and may also contain various folds which help reduce the overall
size of the enclosure. The length of the transmission line
corresponds to the 1/4 wavelength of the resonant frequency of the
driver. The t-line is almost always filled with
various types of stuffing material which help reduce the speed of
sound through the t-line allowing shorter line lengths while still
achieving the proper tuning. For example, a
transmission line for a given driver with an Fs of 25 Hz without
any fill would need to be nearly three times longer than the same
driver in a transmission line that was damped with 8kg/m^3 of
wool. |
|
Advantages |
Transient
response is considered equal to (and sometimes better than) a 2nd
order sealed enclosure and is considered far superior to that of a
vented enclosure. The cut-off rate is somewhat shallower
than a typical sealed enclosure and may be as low as 10 dB/octave
or lower. This results in improved deep bass
performance. Less upper bass coloration due to reduced
impedance peaks. A more pure, cleaner, and deeper bass. |
|
Disadvantages |
Difficult
to design as well as build. A transmission line is every
carpenter's nightmare. Extensive knowledge of wood working
is required. Not all drivers will work well for a TL
enclosures, yet there is no specific guideline for which drivers
may work well. There are no concrete methods or formulas for
designing TL's. It's mostly trial and error. Not
recommended for the novice DIY'er. Enclosure size may
be very large. |
|
Best
Applications |
Where
you've got a ton of room, a ton of time, and a ton of
patience. TL's will work well basically wherever any
of the other alignments would work. It's unique performance
characteristics make it suitable for even the most serious
audiophile. |
Isobaric
- Dual Drivers
|
Basic
Theory
|
Two
drivers are mounted together in an enclosure with a cavity of air
between the two drivers. The drivers must operate in phase
with each other. The cavity of air between the drivers
should be made as small as possible without compromising the
operation of either driver. The modeling for this type
of enclosure is done just as you would any other speaker enclosure
except you take the Vas of the driver and divide by 2. This
will in effect make all your speaker enclosures half as big as
they would normally be for any particular driver. |
|
Advantages |
Improved
sonic bass response. Bass is claimed as being tighter,
faster, more accurate and more pure. Vas of the driver is
cut in half. The volume of enclosure required to obtain a
specific frequency response can be achieved in only half the
volume. This is where isobaric enclosures have their biggest
advantage. |
|
Disadvantages |
Wasted
amplifier power to driver the internal sub. Efficiency of
the system is down 3 dB as compared to a single driver due to the
added cone mass and the reduced Vas. When you compare
isobarics to a system which houses two drivers each in their own
enclosure, this system wis actually 6 dB less
efficient. |
|
Best
Applications |
Where size
is a big issue. When you want the box to be very
small. Where more accurate bass is more important than lots
of bass. If you have a hefty amplifier with plenty of juice
to spare and a driver that can handle a good amount of
power. Suited for music, home theater and car.
Not used too often these days. |
Compound
~ Push/Pull Dual Drivers
|
Basic
Theory
|
Two drivers share an acoustic volume of air within a
single enclosure. The best way to take advantage of this
alignment is to mount one driver facing outwards with the other
driver inverted and facing inwards. The drivers are then
wired so that they are electrically out of phase while remaining
mechanically still in phase with one another. Odd ordered
harmonics are cancelled out by using this approach according to
Vance Dickason. According to M&K who specialize is
push/pull subwoofers claim that this approach cancels out even
ordered harmonics. So take your pick. Either way,
harmonic distortion is reduced in that any anomalies or variations
in the two driver's spider, cone or suspension characteristics are
canceled out by the other driver's inversely proportional
anomalies and variations. The sound is said to be as accurate and pure
as it can possibly be with each driver "correcting" the
other driver. Option designs include having the two drivers share the same acoustic volume of air while maintaining the more
traditional look of having both drivers fire forward into the
listening environment. Though this does not have the
same harmonic cancellation effect, all other characteristics
between the two alignments is identical. Box volume must be
twice that of a single driver. This can be easily modeled by
taking the Vas of a single driver and multiplying it by two.
The system has an increased efficiency of 3dB over a single
driver. Power handling for the system is twice that of
single driver. Frequency response is the same for a single
driver in an enclosure exactly half the size. |
|
Advantages |
Increased output and power handling. Very high
SPL capability. |
|
Disadvantages |
One single huge speaker enclosure that may be both
unattractive and hard to move. Response it essentially
identical to building two smaller enclosures of exactly half the
size but without the versatility of placement of two separate
subs. There are no real disadvantages to building
this kind of enclosure as the speakers will behave just as they
would in enclosures by themselves. It's very common to make MMT style speakers and use the two drivers in the same enclosure. |
|
Best
Applications |
Where one sub just isn't
enough. High power high output applications. If you
choose to do the push/pull configuration, the sonic advantage may
make this sub more suitable for audiophile music and critical
listening experiences. |
|
|
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Disclaimer: This page has been referenced by
numerous sites over the years and has been, to my knowledge, a
fairly useful resource for many DIY speaker builders such as
myself. These theories are merely a short summary of
concepts, ideas, articles, papers, and books that I've read in
addition to some of my own personal experience on the subject of
speaker enclosure design. By no means does this represent a
complete or completely accurate portrayal of all enclosure
theories, pros and cons or best applications. Use this
information at your own risk. |
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